Need some help. Have a ZT1540 Gravely have replaced all three spindles OEM new belt, OEM and blades still continue to "bog" down at times when cutting. Use to cut at #2 level on height then went to #3 and 4 is way too hi for our yard. Any ideas? Sure would appreciate any possible solutions, engine and transmissions work good. cant afford new lawn mower at this time, Thank you.